Why does viewing a hdmi/displayport input on a PC require a capture card?

I bought a cheapo capture card a while back but the quality is pretty terrible

Is there any reason PCs can't just receive HDMI/displayport input from other devices through the same mechanism they output it?

For that matter also is there any reason HDMI can't be run over Ethernet? Afaik ethernet has far more bandwitth than hdmi nowadays